In the catalytic liquid phase polymerization under positive pressure of olefins to polyolefins, such as polyethylene, polypropylene, polybutylene, etc. the reaction mass comprises polyolefin dissolved in liquid olefin monomer. The monomer is recovered from the dissolved polymer by preheating the liquid reaction mass under pressure in a multi-bank preheater to a temperature at which it is flashed off from the polymer (flash point) and is recycled back to the reactor together with fresh liquid olefin monomer feed, after being compressed to a liquid.
Conventional multi-pass, multi-bank, horizontal tubular preheaters have been used to preheat the reaction mass to the flash point of the liquid monomer. Such preheaters comprise horizontal banks of preheater tubes in series, each bank having a bundle or cluster of parallel heat transfer tubes mounted in a shell by means of a pair of tube sheets or plates at opposite ends of the shell with an inlet channel head or chamber at one end of the shell inletting into the inlet ends of the tubes of the cluster and an outlet channel head or chamber at the other end of the shell into which the outlet ends of the tubes exit. The ends of the tubes are sealed in holes in the tube sheets. In most cases, a single baffled outer heating shell (the heating fluid flows through the shell side and the reaction mass flows through the tubes) is used with the bundles or clusters of tubes of each bank located side by side in parallel relation to the others and with a single head at each end of the banks baffled to form an exit head for the tubes of each bank and an entry head for the tubes of the next succeeding bank in the series so that the flow from the exit ends of the cluster of tubes of each bank is baffled to turn about 180.degree. through the head to the entry ends of the tubes of the next cluster.
In these conventional multi bank exchanger units where channel heads are used to direct the flow through each bank or pass in the exchanger unit as aforesaid, it has been found that phase separation occurs in the channel heads as the polyolefin/monomer mixture is heated. The heavy polymer mass drops out of solution in the channel heads and collects against the inlet tube sheet of one of the exchanger banks or passes. As the polymer accumulates in the channel head and against the tube sheet, it restricts the normal flow pattern through the lower tubed area of the exchanger. This in turn forces the gas phase to flow through the upper rows of tubes in the exchanger. The channel head will continue to fill with polymer until the pressure drop across the exchanger in the section where the gas phase is being forced to flow, reaches the point at which the heavy polymer mass is also forced into flowing sluggishly through the lower portions of the exchanger bundle.
In order to solve this problem inherent with the use of such conventional heat exchangers, generally U-shaped heat exchangers with an inlet head at one end and an outlet head at the other end, have been suggested comprising U-shaped heat exchanger tubes mounted in a U-shaped shell by means of tube sheets at each end of the shell and baffle plates mounted in the shell. One leg of the U-shaped tubes corresponds to one conventional bank of parallel tubes and the other corresponds to the next succeeding conventional bank of parallel tubes. In this way the outlet head of the one bank and the inlet head of the next succeeding bank are eliminated and the flow paths through the tubes are continuous and uninterrupted.